The present invention reIates to a boiling water reactor (BWR) and more particularly to fuel bundles and fuel rods used in boiling water reactors.
A boiling water reactor contains a plurality of fuel bundles, each of which contains a plurality of fuel rods. The fuel rods extend from the bottom to the top of the reactor. The lengthwise direction along the rods is the axial direction of the fuel bundle, and the widthwise direction through the rods is the radial direction.
The nuclear fuel in the fuel rods fission, producing heat, and the heat is conveyed from the reactor by water which enters the bottom of the reactor and exits from the top thereof. The water flows within each fuel bundle and also flows through water gaps between the bundles. The water is continuously heated as it flows up through the reactor. The water flowing in the water gaps does not boil significantly, but the water flowing within the bundles eventually boils at some point along the axial direction of the bundle, thereby creating steam and a boiling region in the fuel bundle. The steam produced from the water travelling through the reactor is used to generate power.
The majority of nuclear fissions that occur in the fuel are created by the absorption of thermal (slow) neutrons by the fuel, thereby causing the fuel to fission and release heat. Each fission produces new neutrons which continue a chain reaction. The new neutrons, commonly referred to as fast neutrons, possess a large amount of energy and must be slowed down by a moderating material in order to produce additional thermal neutrons which can cause further fissions.
Since water is a good moderating material, the water flowing in the water gaps and channels is generally used to moderate or slow down the fast neutrons. However, since the water flowing in the channels eventually boils and creates steam as it travels upward through the bundle, the density of the water in the boiling region diminishes, thereby reducing the moderating capability of the water in the boiling region of the bundle. The variation in moderating capability results in more fissions and, therefore, more power being generated in the non-boiling region of the fuel bundle than in the boiling region of the fuel bundle, thereby creating a non-uniform axial power shape. The axial power shape peaks somewhere in the lower half of the fuel bundle in the nonboiling region.
If uncorrected, this non-uniform axial power shape can limit the overall reactor power generation since the excessive temperatures generated in the vicinity where the power peaks must not exceed design limitations. This non-uniform axial power shape can also cause the fuel to be consumed in the bottom of the fuel bundle at a rate faster than it is consumed in the top of the fuel bundle at the beginning-of-life of the fuel bundle. At the end-of-life of the fuel bundle, the lower section of the fuel bundle burns out, but a large portion of the fuel in the top of the fuel bundle remains unburned, resulting in poor fuel utilization. In addition, near the-end-of-life, the amount of reactivity in the top of the fuel bundle is excessively large due to the relatively small amount of fuel burnup therein, thereby reducing the cold shut-down margin. The excessive reactivity in the top of the fuel bundle also makes the effective scram reactivity insertion rate slower since control rods used in scramming the reactor are inserted through the bottom of the reactor.
Finally, the poor neutron moderation in the upper section of the reactor causes, in this section, fast neutron streaming which is a major source of damage to the reactor pressure vessel and internals.
Present BWR designs modify the axial power shape of the fuel bundles in order to minimize these disadvantages. Present techniques rely on the use of burnable poisons and poisoned control rods. Burnable poisons, such as gadolinium, are generally unevenly dispersed in the fuel rods, with a large amount being put in the lower section thereof to absorb neutrons in this section and help flatten the axial power shape of the fuel bundle.
In addition, during reactor operation, poisoned control rods are partially inserted into the reactor through the bottom of the reactor to also absorb neutrons in that section, further flattening the axial power shape of the bundles.
Both of these methods of power shaping have an adverse effect on fuel utilization since neutrons having a potential for producing fissions are parasitically absorbed. In addition, the reactivity in the top of the fuel bundles at the end-of-life still tends to be quite large due to the larger portion of unburned fuel in that section, whereby the scram reactivity insertion rate and cold shut-down margin are adversely affected. Finally, neither of these methods reduce the fast neutron streaming in the upper section of the reactor pressure vessel.
An additional moderating problem present in BWR designs is caused by the radial distribution of the moderating water. As indicated previously, water flows up through channels in the bundles and up through water gaps between the bundles. However, since the water in the water gaps does not boil but the water flowing in the bundles does, the moderating capability of the water flowing outside of the bundles exceeds that of the water flowing within the bundles. This variation in moderating capability produces a non-uniform radial power shape. The amount of power produced by the fuel rods located closest to the non-boiling water gaps is higher than the amount of power produced by the fuel rods located towards the center of the fuel bundle. Again, in order to limit local power peaks, various methods are utilized to limit the power produced in the fuel rods closest to the water gaps. These methods include using uranium of reduced U.sup.235 enrichment in the fuel rods closest to the water gaps and the inclusion of non-boiling water rods in the center of the bundle. The enrichment variations add to the manufacturing complexity and may also reduce fuel utilization.
Gylfe, U.S. Pat. No. 3,145,150 and Vann et al., U.S. Pat. No. 3,178,354 describe fuel rods having short top and bottom caps of moderating material. Although these moderating caps reduce the axial fast neutron leakage from the fuel bundle, their effect on the axial power shape is limited to the very ends of the bundle. They do not significantly modify the axial power shape over the entire length of the bundle.
Other fuel rod and fuel bundle designs are shown in the following U.S. Pat. Nos. 4,127,443; 3,822,185; 3,793,144; 3,274,066; 3,218,237; 3,157,581; 3,146,173; 3,141,829; 3,133,000; 3,119,747; 3,049,487; 3,039,947.